The Yemenis' acknowledgement that the cargo was theirs clarified the situation in one sense, but at the same time complicated things.

The United States has been trying to cultivate Yemen as a key regional ally in its war on terrorism.

It won't be very happy that the Yemenis are continuing to import such weapons when, according to US officials, they had undertaken to stop.

On top of that, according to a US official, Yemen was contacted some days ago and asked about the vessel.

Surveillance success

At that stage, it denied knowledge. That is why the boarding went ahead.

Submarines might have been used to track the ship from North Korea

The Americans are calling this a successful operation. And in a way it was.

It seems US officials have suspected for some time that this ship and its clandestine cargo were heading for Yemen.

They have certainly had it under surveillance since it left North Korea, using, according to one US official, American naval assets.

Those were probably long-range aircraft, ships, and perhaps submarines.

It was also a successful example of multinational cooperation. The international maritime interdiction force has been operating in the region since the attacks of 11 September precisely to intercept suspicious vessels.

The Spanish got involved because they are currently in charge. The other nations operating in the force are Germany and Britain with the frigate HMS Cumberland.

When they found the suspicious cargo, the Spanish called the Americans, who sent personnel from the nearby helicopter assault ship, USS Nassau.

Mess for US

The whole episode also reinforces Washington's view of North Korea as the world's worst missile proliferator.

The Bush administration no doubt believes that it also vindicates their concern about the spread of such weapons.

And US defence officials still insist it was legitimate to stop the ship - it was unflagged and the weaponry aboard was not on the ship's manifest.

But it has become rather embarrassing and messy at the same time that the Americans basically have to hand back the cargo.

The fallout in terms of US-Yemeni relations at this stage is not clear.